The application of millimeter wave/terahertz waves to imaging and radar technology has been hoped for as the wave can transmit through a material and has a high resolution. Since the wavelength is of submillimeter order, the antenna size can be reduced to submillimeter order. An on-chip antenna formed by integrating antennas in a silicon Integrated Circuit (IC) has been extensively studied. However, a transmission loss in a circuit is relatively large because of the high frequency, and thus a technique to suppress a transmission loss in a circuit in the millimeter wave/terahertz wave band is needed. For example, non-patent literature 1 discloses, when a 4×4 planar array antenna is used at a frequency of 100 GHz or higher, an ohmic loss of 10 dB or greater occurs in a power supply network to the antenna formed by a divider and the like.
Metamaterial technology that can control propagation of an ultra-high frequency signal in a space system by designing the refractive index of a material is hoped to realize an ultra-high frequency band space system device. For example, according to non-patent literature 2, the dimension of a gap of a split ring resonator is changed using the split ring resonator to serve as a unit cell of a metamaterial device, so as to shift a resonance frequency. This shift in resonance frequency allows a transmission phase amount to change in a frequency region that exhibits a transmission characteristic with a low loss, thereby controlling propagation of electromagnetic waves transmitted through the metamaterial device. As described above, as a method of forming an ultra-high frequency wave front with a high transmission loss in the circuit, a technique of forming the wave front of an electromagnetic wave in the space system device rather than in the circuit is effective in terms of suppression of a transmission loss.
FIG. 9 shows the structure of a related representative split ring resonator. The split ring resonator includes a conductor 100 made of a metal, and a gap 101 formed in the conductor 100. An incident electromagnetic wave having an electric field component in a y-axis direction parallel to the gap 101 excites and generates an electromotive force in the gap 101 to generate a circulating current Ic. The circulating current Ic becomes the maximum at an LC resonance frequency determined based on a capacitance component and inductive component derived from the gap 101 and conductor 100.
When shifting the resonance frequency by changing a dimension G of the gap 101, the capacitance component decreases as the dimension G becomes larger, and thus the resonance frequency becomes higher, and an electromotive force V1 excited by the incident electromagnetic wave becomes greater. Conversely, the capacitance component increases as the dimension G of the gap 101 becomes smaller and thus the resonance frequency becomes lower, and the electromotive force V1 excited by the incident electromagnetic wave becomes less.
As shown in (a) of FIG. 10, a characteristic is obtained in which the higher the resonance peak intensity (resonance intensity S) and the lower the electromagnetic wave transmittance (Transmittance) becomes, as the dimension G of the gap 101 of a split ring resonator becomes larger, the higher resonance frequency (Frequency), and a lower capacitance C1 of the gap 101. In this regard, as shown in (b) of FIG. 10, with respect to the characteristic of a transmission phase amount (Transmission phase), a split ring resonator with a higher resonance frequency exhibits a larger phase-change characteristic. Therefore, when an operating frequency OF is set in a frequency region that has higher frequencies than the resonance frequency and has a small loss, a wide band characteristic is obtained in which the transmission phase amount distribution remains unchanged even if the frequency is changed.
As described above, by utilizing the method of shifting the resonance frequency of the split ring resonator by changing the dimension G of the gap 101 of the split ring resonator, in the frequency region that has higher frequencies than the resonance frequency and has a small loss, a characteristic is obtained in which the changes in transmission phase amount caused by the change in frequency becomes less, and thus a wide band characteristic of 15% or higher can in principle be implemented in non-patent literature 2.
A metamaterial device using a unit cell having a wide band characteristic of 15% or higher can be used as a condenser/deflection lens in a wireless system in which a fractional bandwidth of about 10% is generally required.
On the other hand, when utilizing a unit cell structure in which the transmission phase amount characteristic strongly depends on an incident electromagnetic wave frequency, a frequency-sweep-type beam steerer with deflection angles that change in accordance with the incident electromagnetic frequency like a prism can be implemented, thereby applicable to a system such as a radar and imaging. However, in the similar unit cell shown in FIG. 9, the resonance intensity becomes higher as the dimension G of the gap 101 becomes larger, and thus, in principle, a transmission phase amount characteristic in a wide band of about 15% is obtained. Therefore, when utilizing a metamaterial device that employs the similar unit cell to serve as a frequency sweep beam steerer, a frequency sweep of at least 15% or more is needed.